Cancer chemotherapy was established with the alkylating agent Cyclophosphamide (Endoxan®), an oxazaphosphorine pro-drug activated preferentially in the tumor. The target of alkylating agents like Cyclophosphamide is DNA and the concept, that cancer cells with uncontrolled proliferation and a high mitotic index are killed preferentially, proved to be very successful. Standard cancer chemotherapeutic drugs finally kill cancer cells upon induction of programmed cell death (“apoptosis”) by targeting basic cellular processes and molecules. These basic cellular processes and molecules include RNA/DNA (alkylating and carbamoylating agents, platin analogs and topoisomerase inhibitors), metabolism (drugs of this class are named anti-metabolites and examples are folic acid, purin and pyrimidine antagonist) as well as the mitotic spindle apparatus with αβ-tubulin heterodimers as the essential component (drugs are categorized into stabilizing and destabilizing tubulin inhibitors; examples are Taxol/Paclitaxel®, Docetaxel/Taxotere® and vinca alkaloids).
A subgroup of proapoptotic anticancer agents target cells preferentially in mitosis. In general these agents do not induce apoptosis in non-dividing cells, arrested in the G0, G1 or G2 phase of the cell division cycle. In contrast, dividing cells going through mitosis (M-phase of the cell division cycle), are killed efficiently by induction of apoptosis by this subgroup agents. Therefore, this subgroup or class of anti-cancer agents is described as cell-cycle specific or cell-cycle dependent. Tubulin inhibitors, with Taxol (Paclitaxel®) as a prominent example, belong to this class of cell-cycle specific, apoptosis inducing anti-cancer agents.